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A functional disorder is a medical condition that impairs normal functioning of bodily processes that remains largely undetected under examination, dissection or even under a microscope. At the exterior, there is no appearance of abnormality. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.
Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or occasionally unimportant for treatment purposes. The brain or nerves are often believed to be involved. It is common that a person with one functional disorder will have others.
Whether a given medical condition is termed a functional disorder depends in part on the state of knowledge. Some diseases, including epilepsy, schizophrenia, and migraine headaches were once considered functional disorders, but are no longer generally classified that way.